New World Screwworm (NWS) is once again a real threat to U.S. livestock, pets, and wildlife, following its northward spread through Central America and Mexico and a confirmed incursion into Texas in 2026. The parasite was eradicated from the U.S. in 1966 through decades of sustained effort, which makes prevention and early detection genuinely critical right now — not routine maintenance, but active vigilance. Here’s what protecting your animals actually involves.
Understand What You’re Protecting Against
NWS is caused by the larvae of the screwworm fly (Cochliomyia hominivorax), which lay eggs in open wounds or body openings. Once hatched, the maggots burrow into living tissue — feeding as they go, tearing flesh with sharp mouth hooks in a motion that gives the parasite its name. Unlike many maggot species that only feed on dead tissue, screwworm larvae attack living flesh, which means wounds can grow larger and deeper as more larvae hatch and feed, causing serious and potentially fatal damage if left untreated. Adult screwworm flies resemble a common housefly but slightly larger, with orange eyes, a metallic blue or green body, and three dark stripes along the back.
Know the Warning Signs
Catching an infestation early makes a significant difference in outcome. Watch for:
- The presence of maggots in wounds or body openings, including the nose, ears, genitalia, or a newborn animal’s navel
- An intense smell of decay, often one of the very first signs before the wound itself is visually obvious
- Wounds that seem to be growing rather than healing, especially ones that were previously minor
Producers are encouraged to physically observe livestock daily where possible, since the smell of decay frequently gives away an infestation before the wound is easy to spot visually.
Treat Wounds Immediately, Every Time
Because NWS specifically targets open wounds, minimizing and treating them promptly is one of the most effective prevention steps available. This includes:
- Treating the umbilical cords of all newborn animals with an approved insecticide immediately after birth
- Treating any wound on any animal right away, rather than waiting to see if it heals on its own
- Delaying non-essential wounding procedures — dehorning, disbudding, branding, shearing, ear notching, castration — during periods of active local risk, since these all create fresh entry points
- Controlling other wound-causing pests, particularly ticks, since any parasite that breaks the skin creates an opportunity for screwworm flies to lay eggs
Keep Facilities and Housing Clean
Reducing breeding grounds around your animals matters as much as treating individual wounds. Regularly clearing waste, uneaten feed, and other organic matter cuts down on conditions that attract screwworm flies in the first place. It’s also worth inspecting animal housing, handling areas, and milking facilities for sharp objects or edges that could injure animals and create new wound sites.
Know What Treatment Options Are Currently Authorized
There are currently no drugs with full, standard FDA approval specifically for NWS myiasis, but the FDA has issued a series of Emergency Use Authorizations (EUAs) and conditional approvals throughout 2026 covering multiple species. Depending on the animal involved, authorized products include topical powders, injectable treatments for cattle, horses, swine, sheep, and deer, and over-the-counter options for dogs and cats. Because authorization status and species coverage continue to shift as the situation develops, talk to your veterinarian directly about which currently authorized product applies to your specific animals rather than relying on older information.
If an animal is suspected of infestation, current guidance calls for treating the wound with an EPA-approved insecticide every two to three days and keeping the animal in quarantine until the wound has fully healed.
Report Suspected Cases Immediately
Because field identification of screwworm larvae can be genuinely difficult, even under a microscope, any suspected case needs to go through official channels for confirmation rather than being assumed and treated informally. If you notice signs of infestation:
- Contact a local accredited veterinarian, your State Animal Health Official, or a USDA veterinarian right away
- In Texas specifically, report suspected livestock cases directly to the Texas Animal Health Commission (TAHC) at your closest area office
- A USDA-accredited veterinarian or state/federal authority will collect samples and send them to the National Veterinary Services Laboratories for definitive identification
Take Extra Precautions If You’re in or Near an Affected Region
If you’re traveling from or through an NWS-affected area, inspect your vehicle for screwworm flies before leaving, since the flies can travel this way into unaffected regions. Every state carries some level of risk given the volume of international travel involving people, pets, and livestock, so vigilance isn’t limited only to states bordering the current outbreak zone.
Official Contacts
USDA APHIS – New World Screwworm Program
Website: aphis.usda.gov (search “screwworm”)
Screwworm.gov – U.S. Government Unified NWS Response
Website: screwworm.gov
Texas Animal Health Commission (TAHC)
For reporting suspected cases in Texas specifically
Website: tahc.texas.gov
FDA Center for Veterinary Medicine
For current information on authorized animal drugs for NWS
Email: [Login to see the link]
Join The Discussion
Are you currently managing livestock or pets in an area affected by the New World Screwworm outbreak, and what prevention steps have you found most effective? Share how you’re handling wound treatment protocols, working with your veterinarian on authorized products, or adjusting your daily inspection routine given the current risk. If you’re new to dealing with this threat, feel free to ask questions — there’s a good chance someone here has firsthand experience navigating it.
